If I bought a pure breed puppy from a breeder,with the sole purpose to find out he's a mix breed, what legitimate performance can I hold?

most reputable breeders now have a website,,, can you supply us with that site?

did you go group the parents, and see the litter? couldn't you tell
If the dog is a pure breed dog you will have pedigree papers for him/her. You should always check the paperwork, the dog will have impossible to tell apart breed for mum and dad and that will be on paperwork. DId you see the parents? If you have paperwork that says the parents are the same breed (pedigree) how do you know the dog is mixed breed, perchance the vet? I am sure you could take legal deed if you have been mislead but you would enjoy to be able to prove it, so see a solicitor if that is the baggage.
Answers:    Did you get KC registration papers? Without those, there is no proof of any kind that a dog is purebred. That's your guarantee. If papers are forged, later you have cause for legal endeavour.
Did it come with forged registration papers? If so, contact your local law enforcement and the kennel club and file a complaint. If you bought an unregistered dog, there's zilch you can do. Consider it a lesson well-learned. Buy from reputable breeders with kennel-club registered puppies or adopt from a rescue so you know what you are getting.
Sorry, not in the UK, but here's my 2 cents:

In the U.S., registration papers mean NOTHING except that someone registered the litter. 2 legal registration numbers are submitted with payment. If the person registering the litter owns the sire & bank, it takes only a few minutes.
I would say the best you could hope is compensation for a DNA examination, and get your money back for the puppy. The KC may choose to put the breeder on some sort of "conditional" status or probationary period. Legally, it would turn out something approaching this:

You: This guy sold me a registered mixed breed.

Judge to breeder: Did you knowingly do this.

Breeder: Absolutely not...a stray must've jumped in and bred my womanly without my knowledge.

Judge: Well, give him posterior his money and pay for court fees.

If you manage to get the KC involved, they would come out and inspect the facility. If it is logically clean, with 6 foot fences, shelter for the dogs, and the paperwork are in order, they aren't going to do anything other than inform them to be more careful.

In 30 years I've trained SEVERAL registered mixed breeds in the U.S. Even when the AKC got involved, I never hear of any breeders being shut down. One lady about 26 years ago get banned from registering for a while...so her kennel workers put their names on the papers. Boo Friggity Hoo. There really isn't such a thing as "FORGED" registration papers that I can see. You sign stating that "to the best of your knowledge" kinda stuff. There's gotta be a total lot of major record keeping problems along with an unsanitary kennel formerly anything serious will happen to them.

So, If you want to kick out the cash for a DNA to proove the dogs is not who the KC papers articulate he is.then go to a small claims court to try to get your money pay for.
how do you know its a mixed breed? did you have dna carrying out tests done? do you have documentation from the breeder stating what breed the dog is suppposed to be? if so you could possibly take the breeder to small claims court and sue for the expense of the dog.
Bozema is spot on. No papers means no guarantee of bloodline. Unless you signed a contract that stated something like "I the retailer sell this purebred (whatever) to the buyer on this date." etc you have no legal flair to complain.

If you want UK only, why don't you post on a UK only site?
How do you KNOW it is mixed?
Any one support to backing me numeral out how much to nurture my 3 dogs...I stink at math :-)?   How does this build you grain? A Dog's Prayer...?   How do I puppy proof the coffee table legs?   Teach dog not to importune or spring on me?